The one I hate the most is when Brits say they are gagging for a drink, be that tea, coffee, beer, wine etc. Sounds to me like they are ready to vomit so I tend to step away from them. Luckily I don't hear it very often.
I also hate the way Americans misuse the word good when they should be saying well, as in 'that pink shirt goes good with that jacket' when it should be 'that pink shirt goes well with that jacket'. (I had a British english teacher when I was in 11th grade and he drilled that one into us)